Well, no more interventionism.Good OP. Until the atomic bomb, the US and UK could not have defeated Germany without the Soviet Union. That being said, Stalin was as (or more) murderous a dictator than Hitler. Regarding Poland, both Germany and the Soviet Union wanted to recover territories they had lost due to WW1. The Poles' recalcitrance on giving Germany territorial access to its East Prussia province gave Hitler an historical argument for invading Poland. Just as with Belgium before WW1, the Brits made empty promises to defend Poland and then used that as an excuse to declare war on Germany. Hitler subsequently sealed his (and Eastern Europe's) fate by attacking the Soviet Union and then declaring war on the United States.
"More murderous" - it's because Stalin defended Russia and didn't let to "civilized countries" to plunder and destroy it? )
Yes, USSR wanted to recover some territories and did it. But talking about "USSR invasion in Poland" is ridiculous, because:
1. USSR forces crossed the border at 17th of September, right after Poland government officially capitulated. Are they invaded in Poland? I'm not sure - Poland didn't existed at this moment. It was an operation at potencially German territory and against German, what's the matter?
2. USSR forced returned territories at the east of Curzon Line - recognized by all world society and Antanta/Britain in particular as ethnically Russian territories.
So, USSR just ratified the solution of world society and started the war with "less murderous dictator", murdered to that moment a several million jews in Holocaust... Where I could be wrong, heh?
Soviet and Nazis in occupied Poland, socializing after joint victory parade.
German–Soviet military parade in Brest-Litovsk - Axis History Forum